Roman Abramovich must show how he acquired his fortune before he receives a new visa allowing him back into the UK, it has emerged.

The oligarch, who owns Chelsea Football Club, has been forced to apply for a new investment visa after letting his previous one expire.

But new rules require him to pass a tougher visa test that includes proving that his funds were obtained lawfully.

The wealth of dozens of oligarchs is being investigated by the UK's National Crime Agency (NCA) as part of a wider crackdown. There is no suggestion of wrongdoing by Mr Abramovich nor that the NCA is delving into his finances. But the Home Office is demanding to know how Mr Abramovich became a billionaire.

His fortune is founded on the Siberian Sibneft oil company, which was privatised in a 1995 auction that some industry analysts believe may have been rigged. The refusal to grant a new visa - or at least delay it - will further raise tensions between London and Moscow.

Mr Abramovich's dealings - like those of other oligarchs in the UK - are under scrutiny following the nerve agent attack on Sergei Skripal and his daughter Yulia in Salisbury in March.

Mr Abramovich's old Tier 1 investor visa, which ran for 40 months, expired while he was abroad, it is understood.

As a result, Mr Abramovich - who was forced to miss Chelsea win the FA Cup on Saturday - had to make an application for a new visa under tougher rules brought in 2015, rather than the more simple process of renewing an existing visa.